Sendmail I/O error with large attachments

Situation: We have seen outgoing mail delivery fail when sending large file attachments to remote SMTP servers. In this example, the sender is attempting to send a Microsoft PowerPoint attachment of approximately 2.6 MB. Also, we were able to send smaller messages to the destination, suggesting that the "I/O error" was not the result of network connectivity problems with the destination.

Certain SMTP servers appear to drop messages over a certain size limit without sending the source SMTP server a helpful message. In this case, the remote SMTP server was running InterScan 3.6 build 1166.

Example message from /var/log/maillog:

Dec 20 13:21:49 mailserver sendmail[12272]: fBKDFXK12188: to=<user@server.com>, delay=00:06:16, xdelay=00:06:02, mailer=esmtp, pri=2712936, relay=mail.server.com. [192.168.1.1], dsn=4.0.0, stat=I/O error

When seeing this message, search for the unique message ID (fBKDFXK12188) in /var/log/maillog, and note the "size=" field in the results. In this case, the size of the message was about 2.6 MB.

We advised the sender to compress the PowerPoint attachment and attempt to resend the message.

Surprisingly, the recipient's domain had two DNS MX records running different mail servers. The lower-preference MX record was running Sendmail 8.9.3, which accepted the large message. The higher-preference MX record was running InterScan 3.6 build 1166, which apparently rejected the message. We saw this because our Sendmail server timed-out with the InterScan server and successfully delivered to the remote Sendmail server.

More information at Google Groups.

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Last modified: 04/18/2002